Dual proton transport in Schiff base-modified chitosan functionalized with AMPS for fuel cell applications

Abstract Proton exchange membranes (PEMs) require high proton conductivity, stability, and durability for fuel cell applications. This study reports the synthesis of N-(2-acrylamido-2-methylpropane sulfonyl) chitosan (CSB) via Schiff base functionalization with 2-acrylamido-2-methylpropane sulfonic acid (AMPS) and the introduction of imine (–CH=N–) and sulfonic acid (–SO 3 H) groups, which significantly enhance dual proton conduction mechanisms through the Grotthuss and vehicular pathways. Structural validation was confirmed by FTIR (new imine stretching peak at 1625 cm −1 ), NMR (imine proton resonance at 8.2 ppm), and XRD [peak shifts from 9.54° to 9.81° for (020) and 20.44° to 19.87° for (110)], increasing the d-spacing from 0.93 to 0.96 Å and 0.43 to 0.47 Å). The BET analysis revealed a surface area of 0.9856 m 2 /g for CSB and 0.3251 m 2 /g for AMPS, with micropore areas of 0.5997 m 2 /g and 0.4955 m 2 /g, respectively, confirming a controlled porous architecture favorable for ion transport. Zeta potential analysis demonstrated the influence of surface charge on stability, with CS exhibiting a strongly positive charge (+ 58.33 mV), AMPS showing near-neutral behavior (+ 0.29 mV), and CSB achieving moderate electrostatic stabilization (+ 5.58 mV). The synergy between the BET micropore distribution and zeta potential regulation enables efficient ion mobility and electrochemical stability, optimizing the proton conductivity. CSB achieved a proton conductivity of 86.2 mS/cm at 100 °C, surpassing that of pristine CS (49.1 mS/cm), with a lower activation energy (10 vs. 24 kJ/mol for CS). Additionally, CSB resulted in lower water uptake (55%) than CS (90%) and reduced methanol permeability (3.276 × 10 −6 cm 2 /s vs. 5.358 × 10 −6 cm 2 /s for CS), ensuring hydration-independent conductivity. Mechanical testing revealed a threefold increase in the tensile strength (31.6 MPa vs. 11.7 MPa for CS) and a significant increase in the elastic modulus (802.4 MPa vs. 195.7 MPa for CS), validating its structural reinforcement ability. These findings confirm the successful incorporation of Schiff base functionalization, demonstrating that CSB is a high-performance, biodegradable alternative to Nafion-based PEMs, offering superior proton conductivity, electrochemical resilience, and mechanical stability for next-generation fuel cell technologies.